The electrolytic production of chlorine and caustic soda by the electrolysis of brine has been well-known for many years. Historically, diaphragm cells using a hydraulically-permeable asbestos diaphragm, vacuum deposited onto foraminous steel cathodes, have been widely commercialized. Such diaphragm cells, employing permeable diaphragms, produce sodium chloride-containing sodium hydroxide catholytes due to the fact that sodium chloride passes through the diaphragm from the anolyte to the catholyte. Such NaCl-containing caustic soda generally requires a de-salting process to obtain a low salt caustic for industrial purposes.
Another type of cell useful for chlorine production is the mercury cell which utilizes a mercury amalgam to remove the sodium. The amalgam is transported to another reactor site where the sodium is reacted with water to form alkali (sodium hydroxide). The electrodes of this invention are useful in mercury cells as they permit use of closer gap without causing shorting and permit more even current distribution than, e.g., the prior art diamond configuration.
More recently, the chlor-alkali industry has focused much attention on developing membrane cells to produce low salt or salt-free caustic in order to improve quality and avoid the costly de-salting procedures. Membranes have been developed for that purpose which are substantially hydraulically-impermeable, but which will permit hydrated sodium ions to be transported from the anolyte portion to the catholyte portion, while substantially preventing transport of chloride ions. Such cells are operated by flowing a brine solution into the anolyte portion and by providing salt-free water to the catholyte portion to serve as the caustic medium. Hydrogen is evolved from the cathode and chlorine from the anode, regardless of whether a membrane cell or a diaphragm cell is employed.
Presently the cost of the electric power which is required to conduct the electrolytic dissociation for the production of chlor-alkali has risen dramatically. The rapid increase in the cost of electric power has in turn spurred a variety of efforts to find ways to lower the amount of electrical energy required to operate chlor-alkali electrolytic cells, thus reducing in turn the cost of the chlorine and caustic soda thereby produced.
Among the various approaches to reducing electric power required to operate electrolytic cells has been the development of the dimensionally stable anode. These anodes customarily are made from valve metal substrates, such as titanium, having a protective coating of a variety of precious or semiprecious metals or metal oxides, e.g., platinum oxide, cobalt spinel, etc. Other efforts have been aimed to reducing the gap or distance between the anode, the cathode and the separating membrane.
These efforts at improving the electrical efficiency of chlor-alkali cells, such as the dimensionally stable anodes, the narrowing in the gaps of electrolytic cells, and others have greatly improved electrical efficiency and utilization.
The present invention enables voltage savings enhanced utilization of the electric power in the chlor-alkali cell, by utilization of electrodes, either anodes, or cathodes, or both anodes and cathodes, having a particular geometric configuration. It is most surprising that the power requirement for conducting the electrolytic dissociation reaction present in a chlor-alkali cell can be improved by controlling the electrode geometry.
Prior to the present invention, it has been customary to utilize anodes of the expanded metal type. By "expanded metal" it is meant that such anodes are produced from metal sheets having varying gauges or thicknesses by cutting or stamping said sheet and then pulling the sheet either in a direction perpendicular to or parallel to the angle at which the cut or punch is made. Thus there have been produced unflattened and flattened expanded metal electrodes with varying shapes, such as diamond hexagonal, etc., when viewed from above (top plan view) and characteristic sectional (side) view configurations depending on the orientation of the section. Electrodes which are made by an expanded metal-type procedure are flatter than others and some, including unflattened electrodes, have fairly sharp edges, which can prove disadvantageous when closely contacting the comparatively delicate hydraulically impermeable membrane. Membranes currently in use are of the polymeric variety, e.g., the membrane material widely employed at present is that developed by the E.I. duPont de Nemours and Co. known in the art as "Nafion.RTM.." This material is a hydrolyzed copolymer of tetrafluoroethylene and a sulfonated perfluorovinyl ether such as is disclosed in U.S. Pat. No. 3,282,875. Demonstrative of unflattened, expanded metal electrodes are the cathodes 10 shown at FIGS. 1-3 in U.S. Pat. No. 4,142,950 to Creamer et al, also contained in an article entitled "Gas Diverting Electrodes in the Chlor-Alkali Membrane Cell" by Jacob Jorne et al appearing in the J. Electrochem. Soc.: ELECTROCHEMICAL SCIENCE AND TECHNOLOGY, February, 1980, as FIGS. 1 and 2. Of similar note is the unflattened electrode shown in plan and respective end views at FIGS. 4, 5 and 6 in U.S. Pat. No. 4,105,514 to Justice et al and described as a louvered mesh cathode. Such unflattened cathodes are typical of the prior art unflattened expanded metal electrodes shown in FIGS. 4-6 herein.